Failed ligaments, such as the anterior or posterior cruciate ligaments in the knee joint, significantly limit physical activity and potentially cause chronic knee problems. The anterior cruciate ligament (hereinafter ACL) and the posterior cruciate ligament (PCL) to a lesser extent are often torn during sports related injuries or as result of traumatic stresses. Ligament reconstruction with allograft and autograft tissue has been shown to improve joint function and provide long term improvement in restoration of physical activity. A common surgical method of repair of an ACL is harvesting a patient's patellar tendon with bone blocks from the tibia and patella. The bone-patellar tendon-bone implant offers several advantages, including high initial tensile strength, stiffness, proper length, rigid fixation and direct bone-to-bone incorporation.
The ACL of the knee functions to resist anterior displacement of the tibia from the femur at all flexion positions. The ACL also resists hyper-extension and contributes to rotational stability of the fully extended knee during internal and external tibial rotation. The ACL may also play a role in proprioception. Structurally, the ACL attaches to a depression in the front of the intercondylar eminence of the tibia extending poster-superiorly to the medial wall of the lateral femoral condyle. Partial or complete tears of the ACL are very common, comprising about 100,000 outpatient procedures in the U.S. each year. The preferred treatment of the torn ACL is ligament reconstruction, using a bone-ligament-bone autograft. Cruciate ligament reconstruction has the advantage of immediate stability and a potential for immediate vigorous rehabilitation. However, the disadvantages to ACL reconstruction are significant: for example, normal anatomy is disrupted when the patellar tendon or hamstring tendons of the patient are used for the reconstruction; placement of intraarticular hardware is required for ligament fixation; and anterior knee pain frequently occurs. Moreover, recent reviews of cruciate ligament reconstruction indicate an increased risk of degenerative arthritis with intraarticular ACL reconstruction in large groups of patients.
A second method of treating ACL injuries, referred to as “primary repair”, involves suturing the torn structure back into place. Primary repair has the potential advantages of a limited arthroscopic approach, minimal disruption of normal anatomy, and an out-patient procedure under a local anesthetic. The potential disadvantage of primary cruciate ligament repair is the perception that over the long term, ACL repairs do not provide stability in a sufficient number of patients, and that subsequent reconstruction may be required at a later date. The success rate of such anterior cruciate ligament repair has generally hovered in the 60% to 70% range.
The autogenous patellar tendon is an excellent tendon replacement source, providing proper tendon length and bone blocks that are fully osteointegrated without immunological rejection. Unfortunately harvesting autogenous bone-tendon-bone (hereinafter B-T-B) also has a number of adverse effects, including donor morbidity (pain), patellar fracture, tendon rupture and degeneration of the patellofemoral articular surface. As an alternate to autogenous graft tissue, synthetic materials have previously received FDA approval. In this regard polyester braids, steel wire and PTFE (GORE-TEX) have been used surgically. All of these materials have failed to integrate into the bone resulting in the tendon's inability to sustain the tensile and torsional loads applied to the knee in normal usage. Nearly all of these synthetic repairs have been revised with autogenous and/or allograft tissue.
There is a limited supply of allograft bone-patellar tendon-bone (B-PT-B) tissue due in large part to the number of donors that qualify according to the selective donor acceptance criteria. As a result of the limited number of available grafts there is a demand for such grafts which exceeds supply.
The use of substitute bone tissue dates back around 1800. Since that time, research efforts have been undertaken toward the use of materials which are close to bone in composition to facilitate integration of bone grafts. Development has taken place in the use of grafts of a mineral nature such as corals, hydroxyapatites, ceramics or synthetic materials such as biodegradable polymer materials. Surgical implants should be designed to be biocompatible in order to successfully perform their intended function. Biocompatibility may be defined as the characteristic of an implant acting in such a way as to allow its therapeutic function to be manifested without secondary adverse affects such as toxicity, foreign body reaction or cellular disruption.
Human allograft tissue is widely used in orthopaedic, neuro-, maxillofacial, podiatric and dental surgery. The tissue is valuable because it is strong, biointegrates in time with the recipient patient's tissue and can be shaped either by the surgeon to fit the specific surgical defect or shaped commercially in a manufacturing environment. Contrasted to most synthetic absorbable or nonabsorbable polymers or metals, allograft tissue is biocompatible and integrates with the surrounding tissues. Allograft bone occurs in two basic forms; cancellous and cortical. Cancellous bone is a less dense structure than that of cortical bone and is also comprised of triple helix strands of collagen fiber, reinforced with hydroxyapatite. The cancellous bone includes void areas with the collagen fiber component contributing in part to torsional and tensile strength.
Many devices of varying shapes and forms are fabricated from allograft cortical tissue by machining. Surgical implants such as pins, rods, screws, anchors, plates, intervertebral spacers and the like have been made and used successfully in human surgery. These pre-engineered shapes are used by the surgeon in surgery to restore defects in bone to the bone's original anatomical shape. At the present time cancellous bone has not been commercially used for shaped devices which are subject to pull out forces.
Allograft bone is a logical substitute for autologous bone. It is readily available and precludes the surgical complications and patient morbidity associated with obtaining autologous bone as noted above. Allograft bone is essentially a collagen fiber reinforced hydroxyapatite matrix containing active bone morphogenic proteins (BMP) and can be provided in a sterile form. The demineralized form of allograft bone is naturally both osteoinductive and osteoconductive. The demineralized allograft bone tissue is fully incorporated in the patient's tissue by a well established biological mechanism. It has been used for many years in bone surgery to fill the osseous defects previously discussed.
U.S. Pat. No. 5,972,368 issued on Oct. 26, 1999 discloses the use of cortical constructs (e.g. a cortical dowel for spinal fusion) which are cleaned to remove all of the cellular material, fat, free collagen and non-collagenous protein leaving structural or bound collagen which is associated with bone mineral to form the trabecular struts of bone. The shaped bone is processed to remove associated non-collagenous bone proteins while maintaining native bound collagen materials and naturally associated bone minerals. The surface of a machined cortical bone surface is characterized by a wide variety of openings resulting from exposure by the machining process of the Haversian canals present throughout cortical bone. These canals serve to tansport fluids throughout the bone to facilitate the biochemical processes occurring within the bone. They occur at variable angles and depths within the bone.
In French Patent Applications Numbers 2,582,517 and 2,582,518 fragments of bones taken from animals, primarily cattle were partially demineralized and tanned with glutaraldehyde. The bone elements to be implanted were cut to the desired shape from an ox bone which has been subjected to a treatment comprising a degreasing step with an organic solvent such as ethanol, a demineralization step with a calcium extraction agent such as hydrochloric acid and tanning with glutaraldehyde and subsequent washings. Similar demineralization of bone is shown in U.S. Pat. No. 5,585,116 issued Dec. 17, 1996 where partial demineralization is used to facilitate integration of a bone graft. This is accordingly followed by different complementary steps which are intended either to deproteinize the bone completely or to act on the nature of the proteins which then remain linked within the bone matrix or else to increase the proportion of proteins.
Much of the structure and many of the properties of original tissues may be retained in transplants through use of xenogeneic or heterograft materials, that is, tissue from a different species than the graft recipient. For example, tendons or ligaments from cows or other animals have been covered with a synthetic mesh and were transplanted into a heterologous host in U.S. Pat. No. 4,400,833. Flat tissues such as pig pericardia are also disclosed as being suitable for heterologous transplantation in U.S. Pat. No. 4,400,833. Bovine peritoneum fabricated into a biomaterial suitable for prosthetic heart valves, vascular grafts, burn and other wound dressings is disclosed in U.S. Pat. No. 4,755,593. Bovine, ovine, or porcine blood vessel heterografts are disclosed in WO 84/03036. However, none of these disclosures describe the use of a xenograft for ACL replacement.
Xenograft materials must be chemically treated to reduce immunogenicity prior to implantation into a recipient. For example, glutaraldehyde is used to cross-link or “tan” xenograft tissue in order to reduce its antigenicity, as described in detail in U.S. Pat. No. 4,755,593. Other agents such as aliphatic and aromatic diamine compounds may provide additional cross linking through the side chain carboxyl groups of aspartic and glutamic acid residues of the collagen polypeptide. Glutaraldehyde and diamine tanning also increases the stability of the xenograft tissue.
U.S. Pat. No. 5,562,669 issued Oct. 8, 1996 discloses a B-T-B tendon anchor device using autologus bone plugs taken from the cores drilled out from the bone tunnels of the patient or alternatively donor bone, namely allograft bone to make the bone plugs. The linear cylindrical plug member is provided with two longitudinal substantially parallel grooves cut on opposite sides of each bone plug which provide a recess in which the tendon can be seated. A notch may also be drilled if desired across one end of the bone plug so that the tendon can be wrapped alongside and around the end of the bone plug without protruding excessively from the plug. Suture holes can be cut through the bone plug for attaching the tendon to the plug as is shown in FIGS. 4a and 4b. The perfectly symmetric pattern of the '669 device presents the tendon equally on both sides of the bone block.
Likewise U.S. Pat. No. 5,632,748 issued May 27, 1997 discloses a B-T-B tendon anchor device formed of plastic, bone, stainless steel or any other suitable material. The body is tapered and formed with a groove to receive a fixation screw and two curved recesses to hold a tendon which is looped over the device. The fixation groove is provided with threads (FIG. 3) and the tendon grooves are provided with teeth. (FIG. 4). A two piece version having a tongue and groove and stepped mating faces for joinder with two tendon grooves is shown in FIG. 7.
U.S. Pat. No. 6,264,694 issued Jul. 24, 2001 discloses a spherical member having a through going bore and parallel recessed surfaces which enable it to be tied to the end of a soft tissue ligament graft to allow the graft to be secured within the bone tunnel by an interference screw.
Presently, the bone block systems in B-T-B grafts have been made from cortical bone or synthetic materials. Cancellous bone has not been used for bone tendon bone assemblies because of concerns with strength and screw pullout.